email_mirror: Add a test for sending to a private stream.

This verifies an important case.  We still have an open bug for why in
some production environments, the email_gateway_bot seems to not be
tagged as an API super user (resulting in this code path not working).

zerver/tests/test_email_mirror.py

@@ -125,6 +125,31 @@ class TestStreamEmailMessagesSuccess(ZulipTestCase):
         self.assertEqual(get_display_recipient(message.recipient), stream.name)
         self.assertEqual(message.topic_name(), "(no topic)")
 
+    def test_receive_private_stream_email_messages_success(self) -> None:
+        user_profile = self.example_user('hamlet')
+        self.login(user_profile.email)
+        self.make_stream("private_stream", invite_only=True)
+        self.subscribe(user_profile, "private_stream")
+        stream = get_stream("private_stream", user_profile.realm)
+
+        stream_to_address = encode_email_address(stream)
+
+        incoming_valid_message = MIMEText('TestStreamEmailMessages Body')  # type: Any # https://github.com/python/typeshed/issues/275
+
+        incoming_valid_message['Subject'] = 'TestStreamEmailMessages Subject'
+        incoming_valid_message['From'] = self.example_email('hamlet')
+        incoming_valid_message['To'] = stream_to_address
+        incoming_valid_message['Reply-to'] = self.example_email('othello')
+
+        process_message(incoming_valid_message)
+
+        # Hamlet is subscribed to this stream so should see the email message from Othello.
+        message = most_recent_message(user_profile)
+
+        self.assertEqual(message.content, "TestStreamEmailMessages Body")
+        self.assertEqual(get_display_recipient(message.recipient), stream.name)
+        self.assertEqual(message.topic_name(), incoming_valid_message['Subject'])
+
 class TestStreamEmailMessagesEmptyBody(ZulipTestCase):
     def test_receive_stream_email_messages_empty_body(self) -> None: